Regional organizations (ROs) display significant variation in their institutional design. Some have a diversified institutional architecture; others are fairly simple in their institutional organization. Some make decisions by consensus; others use majoritarian decision-making rules. Some appear to be relatively fixed in their institutional structure, while others change considerably over time. This chapter addresses three key questions related to the institutional design of ROs: What are the principal empirical patterns? How can design variation be explained? And how is it related to states’ ability to achieve collective goals? The chapter suggests that pooling and delegation capture distinct aspects of regional organization and examines how the literatures on realism, institutionalism, constructivism, and diffusion explain the variation that can be detected. The authors then review the consequences of institutional design for peace and security, economic welfare, domestic institutions, and international actorness. They conclude by discussing some promising avenues for future research.